When the excimer laser or fluorine molecule F2 laser is used as a light source for a stepper (reduction projection exposure device), it is necessary to make the oscillation laser beam of the excimer laser to have a narrow band. Furthermore, it is necessary to make the stabilization control with high accuracy so that the central wavelength of the spectrum of the narrowbanded oscillation laser beam does not displace while exposing.
FIG. 34 shows a general laser wavelength stabilization controller.
Narrowbanding is conducted by driving a narrowbanding element such as an etalon and a grating provided in narrowbanding module 26 via driver 46 by controller 20 (e.g., adjusting a set angle of the etalon or the grating). When exposing, the wavelength is controlled so that the central wavelength of the spectrum does not vary.
Specifically, an absolute wavelength of oscillation laser beam LO is detected by constantly detecting a relative wavelength of the oscillation laser beam LO to a reference light in monitor module 22 when exposing.
Then, a detected result is fed back to the controller 20, and the narrowbanding element is driven via the driver 46.
And, the central wavelength of the spectrum of the laser beam LO emitted through laser chamber 27 is fixed at a target wavelength.
Japanese Patent Applicaiton Laid-Open No. 4-163980 uses an emission line of arsenic (As) having a wavelength of 193.696 nm as a reference light for detecting an absolute wavelength of the oscillation laser beam of an argon fluorine excimer laser (a wavelength of about 193.3 nm).
Specifically, the emission line from an arsenic-enclosed discharge lamp is entered a spectroscope as a reference light. At the same time, the oscillation laser beam of the argon fluorine excimer laser is entered the same spectroscope as a light to be detected (subject light) for its wavelength. And, the spectroscope separates the entered lights into the reference light and the subject light, and the separated light images are formed on a line sensor. Detection positions on the line sensor correspond to detection wavelengths.
A relative wavelength of the subject light to the reference light is determined from a difference between the detection positions on the line sensor by using a dispersion value. And, the absolute wavelength of the subject light is calculated on the basis of the determined relative wavelength and the wavelength of the known reference light.
Now, the dispersion value will be described.
FIG. 35 shows a relation between a sensor channel number (positions on the line sensor) and a sensor signal strength of the line sensor. The line sensor has a plurality of light-receiving channels and determines the light-detecting positions on the line sensor according to the channel number having detected a light of maximum intensity. The wavelength of the light can be detected from the light-detecting position of the line sensor because an incidence position on the line sensor is variable depending on a wavelength. Thus, the wavelength of light is determined from the channel number having detected the light.
The dispersion value indicates a wavelength (in nm) corresponding to a channel space (in .mu.m) of the line sensor. When the dispersion value (a wavelength corresponding to a channel space of the line sensor) can be determined, a relative wavelength of a light to be detected (subject light) LO to reference light La can be determined from a difference between channel number Sa having detected the reference light La by using this dispersion value and channel number SO having detected the subject light LO.
Correspondence (dispersion value) of the channel space and a wavelength of what level is determined depending on the characteristic of the spectroscope which guides the light onto the sensor. The characteristic of the spectroscope depends on the number of grating grooves, a focal distance of a concave mirror, a refractive index of light in the air and other characteristic values of various types of optical elements configuring the spectroscope.
Here, it was conventionally assumed that the characteristic of the spectroscope was known, namely the dispersion value was a known value, to calculate wavelength .lambda.O of the subject light.
Specifically, theoretical (design of the spectroscope) dispersion value Dt (a wavelength of each channel of the sensor) is determined for each spectroscope on the basis of a design value of the focal distance etc. of the concave mirror in the spectroscope.
And, the obtained theoretical dispersion value Dt is fixed, a relative wavelength of the subject light LO to the reference light La is determined from a difference between the detected channel number Sa of the reference light La and the detected channel number SO of the subject light LO. And, the wavelength .lambda.O of the subject light LO is calculated from the determined relative wavelength and the known wavelength .lambda.a (193.696 nm) of the reference light La.
But, the designed characteristic value of the spectroscope is slightly different from the characteristic value of each produced spectroscope. In other words, the theoretical dispersion value Dt contains an error depending on an individual difference among the spectroscopes.
Moreover, the characteristic of the spectroscope is variable depending on a change in the measurement environment such as a change in temperature and a change in pressure. For example, a space between the grating grooves is varied when a temperature changes. And, a refractive index of light in the air is varied due to a change in pressure. Therefore, the relation between the detection position on the sensor and the wavelength is varied due to the change in temperature and pressure.
As described above, actual dispersion value D of the spectroscope indicates a value different from the theoretical dispersion value Dt due to a difference between the designed characteristic of the spectroscope and the actual characteristic of each produced spectroscope and a change in characteristic of the spectroscope due to the change in the measurement environment. Therefore, on the assumption that the characteristic of the spectroscope is known, namely the theoretical dispersion value Dt is known, the relative wavelength of the subject light LO to the reference light La is determined from a difference between the detection channel number Sa of the reference light La and the detection channel number SO of the subject light LO. And, when the wavelength .lambda.O of the subject light LO is calculated from the obtained relative wavelength and the known wavelength .lambda.a (193.696 nm) of the reference light La, the obtained wavelength .lambda.O includes a detection error. The wavelength .lambda.O is required to have a detection accuracy of 0.0001 nm order, but such a requirement cannot be fulfilled.
The present invention was achieved in view of the above circumstances. And, it is a primary object of the invention to enable error-free and very accurate detection of a wavelength of the subject light output from the subject light source even if the characteristic of the spectroscope is varied due to the individual differences among the spectroscopes or the changes in the measurement environment.
When the subject light is a fluorine molecule F2 laser, a relative wavelength of the subject light LO to the reference light cannot be determined by using the dispersion value as described above because the reference light which can be used to stabilize the wavelength is not known yet.
Therefore, when the fluorine molecule F2 laser was used for exposing, it was hard to control so that the central wavelength .lambda.O of the spectrum of the oscillation laser beam LO does not vary.
The invention was achieved in view of the above circumstance, and it is a second object of the invention to prevent the central wavelength .lambda.O of the spectrum of the oscillation laser beam LO from varying when exposing by the reference light even when the fluorine molecule F2 laser is used as the subject light.